This invention relates to a furnace intended for the heat treatment of elongate materials such as wire, rod, ribbon and the like, particularly materials with a high melting point such as tungsten or molybdenum.
A conventional heating furnace for quickly heating elongate materials, for example, wires at a high temperature is the type which comprises a cylindrical furnace, a plurality of burners arranged around the peripheral wall of the furnace and a plurality of exhaust ports formed in the peripheral wall of the furnace so as to face the plural burners. With this type of heating furnace, wire is directly heated by the flames of the burner while it travels through the furnace and exhaust gas is discharged from the exhaust ports. However, the heating furnace known to date has the drawback that its inside space is not effectively utilized. In other words, the heat supplied from the burners is immediately drawn off to the open air through the exhaust ports, and is retained in the furnace only for a short length of time. Therefore, the heat fails to be fully utilized resulting in an uneconomical operation. A further problem with the conventional furnace is that since the flames of the burner directly touch the wire, the surface of the wire is oxidized, leading to the deterioration of its quality.
To resolve the above-mentioned drawbacks, another heating furnace has been proposed which is so designed that a combustion gas is carried into a cylindrical furnace body from a burner arranged at one end of the furnace body. The combustion gas heats the wire held in the furnace body and is drawn off through an exhaust port provided at the opposite end of the furnace body. This proposed heating furnace offers the advantage that heat is retained for a longer length of time in the furnace body than in the above-mentioned conventional furnace. When said proposed furnace is applied to the heat treatment of wires having a high melting point such as tungsten or molybdenum, the interior of the furnace body can be heated to a temperature higher than 1,200.degree. C. Therefore, the wall of the furnace body should be made sufficiently thick to withstand the thermal shock resulting from such a high temperature. In such case, however, the length of time required to heat the furnace body to a prescribed temperature is extended, leading to the consumption of the combustion gas. Moreover, the above-mentioned thick walled furnace is regarded as unadaptable for high heat efficiency and consequently is regarded as uneconomical.